Improve NormalizeData

Initialize NormalizeData with data columns
``` python
with snt.NormalizeData(xaxis=True, yaxis=True, xdata, ydata) as n:
```

If zdata is a scalar, following error ocures:
``` bash
    zminpos = np.nanmin(vdata[vdata>0]) if np.sum(vdata>0) else np.nan
TypeError: '>' not supported between instances of 'list' and 'int'
```

Ideally, one would set x and y first and then the z data values